1. Field of the Invention
The present invention relates generally to devices and methods for treating spinal disorders and more specifically to a variable volume inter-spinous process spacer that can be implanted in a minimally invasive manner.
2. Description of the Background
Degeneration of the intervertebral discs and the concomitant instability and translocation of the vertebra (spondylolisthesis) is a common cause of back pain and may result from a variety of problems including congenital deformity, age related degeneration, osteoporosis, tumor and disc herniation as a result of trauma. Disc degeneration, for whatever reason, results in compression of the spinal nerve roots (stenosis) resulting in pain. Other causes of stenosis include changes in ligament morphology, vertebral bone spurs in or near the spinal canal and degeneration of the facet joints. Palliative care such as physical therapy, nonsteroidal anti-inflammatories (NSAIDS) and epidural steroid injections are often successful used in the treatment of mild cases but more extreme or degenerative cases may require a surgical approach to stabilize the spine and relieve pressure.
The standard surgical treatments for symptomatic degenerative spondylolisthesis and spinal stenosis have, for many years, included decompressive laminectomy in which the lamina of one or more vertebrae is removed to enlarge the space available and relieve pressure on the spinal cord or spinal nerve roots. Lumbar decompression and spinal fusion to decompress the nerve roots and/or spinal cord and to stabilize the spine are also frequently employed. However, many patients suffering from degenerative conditions have comorbidities that make them less than perfect surgical candidates. As a result, several different methods have more recently been devised as alternative treatments for degenerative spondylolisthesis and spinal stenosis.
One such alternative treatment is interspinous process distraction (IPD) which is also known as interspinous distraction or posterior spinal distraction. During IPD the spinous processes are mechanically pushed apart or distracted to relieve pressure on the spinal cord and/or nerve roots that is caused by the spondylolisthesis and spinal stenosis. An inter-spinous process spacer may be inserted into and retained in the interspinous process space created by the procedure to maintain the modified geometry.
The overall goals of traditional lumbar decompression with or without lumbar fusion and IPD are the same in that they both aim to relieve lower extremity neuropathy and claudication and may alleviate low back pain. IPD, however, offers advantages over the standard treatment in that it is not as invasive as laminectomy or lumbar decompression and spinal fusion leading to shorter surgery times and shorter hospital stays and rehabilitation periods. IPD can further be accomplished under local anesthesia, preserves more local bone and soft tissue, has a reduced risk of epidural scarring and cerebrospinal fluid leakage and is reversible so as not to limit any future treatment options. The potential complications of IPD include dislodgement of the spacer, incorrect positioning or sizing of the spacer, fracture of the spinous process, foreign body reaction to the spacer (e.g., allergic reaction to titanium alloy) and mechanical failure of the spacer.
It would, therefore, be an improvement in this art to provide an interspinous process spacer that can be implanted in a minimally invasive procedure so as to retain the advantages of existing methods but that avoids the limitations of previous spacers. Such a spacer would be capable of being securely retained in place, would maintain the greatest possible contact area with the spinous processes so as to distribute loading forces and reduce the chance of fracture, would be of a robust design that is unlikely to fail mechanically and constructed of materials that are unlikely to cause allergic reactions. Such a spacer would further be capable of being sized and positioned during the implantation procedure and of being resized by percutaneous methods after implantation for an extended or indefinite period.